Method for cleaning and passivating a metal surface

ABSTRACT

A surface of a light-metal alloy is anodically polarized in a cleaning process. The surface is cleaned with a solution containing phosphoric acid, alcohol, and optionally fluoride. Additionally, the surface is passivated in an oxidizing process using, for example, fluoride ions or an aqueous oxidizing agent. Preferably, the light-metal alloy has a relatively high Al or Mg content. This method is suitable for preparing the surface for a subsequent coating operation.

BACKGROUND OF THE INVENTION

[0001] This invention relates to a methods for cleaning and preserving a metal surface, particularly a light-metal alloy surface. This invention further relates to methods for preparing a metal surface for subsequent coating processes.

[0002] Many known methods for cleaning light-metal alloy surfaces have the disadvantage that they require a relatively large number of consecutive treatment steps and thus entail a comparatively high degree of complexity and cost. In other cases, these known methods lead to an unsatisfactory cleaning action as far as certain substances are concerned. For example, some processes do not reliably remove polysilanes.

[0003] After undergoing a cleaning procedure, surfaces of light-metal alloys may be passivated, and thus be preserved to a certain extent, by means of known oxidizing treatments. In its general chemical context, such oxidation includes reaction with oxygen, typically used on alloys with a high Al content, as well as reaction with fluoride ions, typically used on alloys with a higher Mg content.

[0004] Some of the known cleaning and passivating methods employ substances which pose a health risk, such as nitric acid, which may release nitrous gases. Furthermore, using conventional methods, it is difficult to ensure the quality of a cleaned surface prior to passivation if the cleaning and passivating steps are combined.

SUMMARY OF THE INVENTION

[0005] Among the several objects of the present invention, therefore, may be noted the provision of a method for cleaning and passivating a light-metal alloy surface which is effective in the sense that it leads to good cleaning properties and is resistant to the composition of the alloy and in the sense that it is also economically efficient. A further object of the present invention is the provision of a method for cleaning and passivating a light-metal alloy surface in preparation for a subsequent plating process.

[0006] Briefly therefore, the present invention is directed to a method for cleaning a metallic surface in which a surface of a light-metal alloy is cleaned and passivated. The cleaning is performed with a solution comprising phosphoric acid and an alcohol, and the surface is passivated in an oxidizing step. The invention is further directed to a method for coating a metallic surface in which a surface of a light-metal alloy is cleaned and passivated prior to coating. The cleaning is performed with a solution comprising phosphoric acid and an alcohol, and the light-metal alloy surface is passivated in an oxidizing step.

[0007] Other objects and features of the present invention will be in part apparent and in part pointed out hereinafter.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0008] In metallurgy and materials science, light-metal alloys are understood to encompass a variety of metal mixtures comprising “light metals,” such as Al, Be, Mg, and Ti. The most common light-metal alloys contain Al or Mg. Preferably, the method according to this invention is performed on light-metal alloys with a relatively high Al content or those with a relatively high Mg content.

[0009] It has been discovered that an anodic cleaning method according to the present invention ensures a very thorough and, at the same time, broad cleaning action of light-metal alloy surfaces. Owing to the anodic operation, the cleaning method according to the present invention has a certain inherent inhibiting effect in that the anodic oxygen reactions of the light-metal surface prevent an excessive erosion of the material.

[0010] Useful anodic current densities on the anodically polarized light-metal alloy surface range from about 50, 30, or even 10 A/m² as the lower limit to about 500 A/m² as the upper limit. An optimal current density is a function of the composition of the alloy, the justifiably acceptable material erosion, and the cleaning action required. It is possible to optimize the overall cleaning and passivating process through anodic current density without changing the composition of a treatment solution selected to passivate a light-metal alloy in an oxidation step. Thus, it is possible to obtain optimum results for different alloys with the same solution composition. It is, of course, also possible to optimize the composition of the solution according to the alloy being treated.

[0011] The solution used in the anodic cleaning steps comprises phosphoric acid in an amount which preferably ranges from about 30 to about 90 percent of the solution on a volumetric basis. Within this range of volume fractions, the phosphoric acid can measure from about 50 to about 95 percent H₃PO₄ by weight. The solution further comprises an alcohol and, optionally, fluoride ions.

[0012] The solution process temperature preferably ranges from about 10 to about 40° C. The overall treatment time for the anodic cleaning steps (if there are several) may range, for example, from about 10 seconds to about 5 minutes and depends to a considerable degree on the current density selected, the justifiably acceptable material erosion, and the degree of soiling.

[0013] In one embodiment, a light-metal alloy surface is cleaned and passivated by means of an oxidation step, characterized by a treatment step with a solution which comprises phosphoric acid and an alcohol, and in which the surface is anodically connected. Useful alcohols comprise common alcohols, such as methanol, ethanol, propanol, butanol, and polyhydric alcohols and derivatives therefore, such as isopropanol. Diols, polyethers, and other alcohols are also useful. Butanol and isopropanol are preferred alcohols. Mixtures of two or more alcohols are also useful.

[0014] The anodic cleaning step with a solution comprising phosphoric acid and alcohol is highly effective both with respect to degreasing and etching the surface and, moreover, is also able to remove problem residues, such as polysilane release agents. For example, when light-metal alloys are produced by means of a die-casting process, they are contaminated with release agents. Even these agents are reliably and completely removed when the cleaning method disclosed by this invention is used.

[0015] The cleaning action of the method according to the present invention is sufficiently thorough and broad that preliminary chemical treatment steps prior to placing a light-metal alloy into the solution comprising phosphoric acid and the alcohol are unnecessary. Thus, the surfaces to be treated can be dry and can be placed directly into the solution.

[0016] In another embodiment fluoride ions are used to passivate the surface. Accordingly the solution used to treat the light-metal alloy surface comprises phosphoric acid and fluoride ions, wherein the light-alloy surface is anodically connected during this treatment step. Useful sources of fluoride ions comprise alkali fluoride, ammonium-(bi)-fluoride, and hydrogen fluoride.

[0017] Treatment with a fluoride-containing solution is particularly useful for light-metal alloys with a substantial Mg content. The treatment step with fluoride ions is preferred for a light-metal alloy having a Mg content of at least about 50 percent by weight. MgF₂ forms as the passivating layer or in the passivating layer.

[0018] Treatment with fluoride ions is also preferred for light-metal alloys with a Si content, more preferably for light-metal alloys having a Si content of at least about 0.1, 0.5, 1 or 2 percent by weight or more. Alloys with relatively low Si concentrations are preferably treated with solutions having relatively low fluoride concentrations. A treatment step with the fluoride ions is still useful for light-metal alloys with a low or negligible Mg content.

[0019] Solutions having fluoride concentrations as low as about 0.5, 0.3, or even 0.1 percent by weight have been found to be useful, as have solutions having fluoride concentrations as high as 10, 20, or even 30 percent by weight.

[0020] Combinations of the above embodiments may be performed. For example, in a third embodiment, the treatment step is performed with a solution comprising phosphoric acid, an alcohol, and fluoride ions. Alternatively, the aforementioned embodiments may be performed separately in succession. For example, a first treatment step performed with a solution comprising phosphoric acid and an alcohol may be followed by a second treatment step performed with a solution comprising phosphoric acid and fluoride ions. To optimize cleaning properties, the solution in the second treatment step may further comprise an alcohol. This alcohol may be the alcohol used in the first treatment step or a different alcohol.

[0021] In another embodiment the cleaning and passivating method according to the present invention further comprises an additional passivating/oxidizing step with an aqueous oxidizing agent. This oxidizing agent may be, for example, a persulfate solution or a solution of peroxomonosulfuric acid (Caro's acid). This additional passivation/oxidation step is preferably performed wherein the light-metal alloy has a substantial Al content, in particular an Al content of at least about 60 percent.

[0022] It is preferred that other treatment steps, e.g. a treatment step with a solution comprising phosphoric acid and especially a treatment step with a solution comprising fluoride ions, are performed before an additional passivation/oxidation step.

[0023] An additional passivation/oxidation step is not necessary for a surface of a light-metal alloy with a high Mg content which is coated with fluoride. The fluoride passivation layer may be harmed by such a step if it is carried out in an excessively acidic range, for example at a pH of no greater than about 6.

[0024] A special advantage of the methods according to the present invention is that the final oxidation of the Al alloy can be carried out without having to use nitric acid since the metal surface is untarnished. Thus, nitrous gases, such as are generated in conventional processes do not form. Accordingly there is not need to employ technically complex procedures for exhaustion and purification of the waste gases, and it is not necessary to obtain approval according to the relevant applicable regulations, e.g. Bundes-Immissionsschutzgesetz, the German Federal Pollution Control Act.

[0025] The cleaning and passivating method according to the present invention may further comprise an alkaline rinsing step performed in alkalized water, for example. Preferably, alkalized water with a pH value at least about 10 is used. An alkaline rinsing step is especially beneficial if the passivating surface is dominated by MgF₂ at preferred pH values but less beneficial if the passivating surface is dominated by Al₂O₃.

[0026] A special advantage of this invention is that it is possible to obtain good results even on regenerated light-metal alloys. In particular, no sludge is generated. In conventional processes, the metal impurities of regenerated materials have led to considerable problems with cleaning and have frequently made it completely impossible to clean and subsequently coat the surfaces. Even if the Al content is relatively high, the metal surfaces in the anodic baths according to the present invention remain untarnished, allowing the subsequent oxidation mentioned earlier without having to use nitric acid.

[0027] Since the quality of a coating depends largely on the cleanliness of the surface, both as far as the appearance of the coating and the load-bearing capacity of the coating are concerned, this invention is particularly useful to prepare light-metal alloy surfaces for subsequent coating steps of any type.

[0028] In particular, another embodiment comprises a subsequent metal plating step, preferably electroless metal plating. The components thus treated can also subsequently be chemically zinc-plated, nickel-plated, or copper-plated, or they can be coated with alloys thereof. In chemical conversion coating, the passivating layers are dissolved or converted, thus ensuring that a good and direct contact between the metals is obtained.

[0029] The following examples illustrate the invention.

EXAMPLE 1

[0030] Without any preliminary chemical cleaning, a dry, anodically polarized AZ91 alloy, an alloy with a relatively high Mg content, is introduced into a bath consisting of 60% phosphoric acid (H₃PO₄) and 40 vol % of butanol. The current density is, for example, 20 A/m² at a temperature of 25° C. and a treatment time, for example, of 30 sec.

[0031] The AZ91 alloy is subsequently introduced into a second bath which has a composition identical to the composition mentioned above but which, in addition, also comprises 2 percent by weight of ammonium bifluoride. A second anodic cleaning is carried out for another 20 sec at the same current density.

[0032] Subsequently, the AZ91 components are rinsed in alkalized water (with a pH value slightly higher than 10). The AZ91 surface is now passivated with a fluoride coating and can be metal-plated using a conventional method. In this particular practical example, chemical conversion coating with Zn, Ni, or Cu or with an alloy made thereof is used. Other alloys with a relatively high Mg content, such as AM50 or AZ31 alloys, may be treated according to the methods presented in this example.

EXAMPLE 2

[0033] An technical aluminum alloy with a high Al content, in this case GdAlSi₈Cu₃, is used. Since this alloy comprises Si, the fluoride bath described in the first practical example is used here as well. The same quantitative parameters can be used, except that the rinsing step in alkalized water is omitted. Instead, neutral water having a pH of approximately 7 is used for rinsing, which is followed by an additional oxidation with persulfate solution to strengthen the passivating layer. This treatment can also be used, for example, for GdAlSi₉Cu₃.

[0034] In view of the above, it will be seen that the several objects of the invention are achieved.

[0035] As various changes could be made in the above material and processes without departing from the scope of the invention, it is intended that all matter contained in the above description be interpreted as illustrative and not in a limiting sense. 

What is claimed is:
 1. A method for cleaning and passivating a metallic surface comprising the steps of: cleaning a surface of a light-metal alloy with a solution comprising phosphoric acid and an alcohol, wherein the alloy is anodically polarized; and passivating the surface in an oxidizing step.
 2. The method according to claim 1 wherein the cleaning solution has a phosphoric acid content which ranges from about 30 to about 90 percent by volume.
 3. The method according to claim 2 wherein the phosphoric acid comprises H₃PO₄ in a concentration ranging from about 50 to about 95 percent by weight.
 4. The method according to claim 3 wherein the cleaning solution consists essentially of phosphoric acid and alcohol.
 5. The method according to claim 1 further wherein the surface is passivated by contacting the surface with fluoride ions.
 6. The method according to claim 5 wherein the light-metal alloy has a Si content of at least about 0.1 percent by weight.
 7. The method according to claim 5 wherein the light-metal alloy has an Mg content of at least 50 percent by weight.
 8. The method according to claim 5 wherein the cleaning solution further comprises fluoride ions.
 9. The method according to claim 8 wherein the cleaning solution has a fluoride content which ranges from about 0.1 to about 10 percent by weight.
 10. The method according to claim 5 wherein the cleaning solution has a phosphoric acid content which ranges from about 30 to about 90 percent by volume.
 11. The method according to claim 10 wherein the phosphoric acid comprises H₃PO₄ in a concentration ranging from about 50 to about 95 percent by weight.
 12. The method according to claim 11 wherein the cleaning solution consists essentially of phosphoric acid, alcohol, and fluoride.
 13. The method according to claim 2 wherein the surface is contacted with a second solution comprising phosphoric acid and fluoride ions subsequent to the cleaning step.
 14. The method according to claim 13 wherein the second solution further comprises an alcohol.
 15. The method according to claim 13 wherein the solution has a fluoride content which ranges from about 0.1 to about 10 percent by weight.
 16. The method according to claim 13 wherein the solution has a phosphoric acid content which ranges from about 30 to about 90 percent by volume.
 17. The method according to claim 13 wherein the phosphoric acid comprises H₃PO₄ in a concentration ranging from about 50 to about 95 percent by weight.
 18. The method according to claim 1 wherein the light-metal alloy has an Al content of at least about 60 percent by weight and wherein the surface is passivated by oxidizing the surface with an aqueous oxidizing agent subsequent to the cleaning step.
 19. The method according to claim 1 further comprising a final alkaline rinsing step.
 20. The method according to claim 1 wherein the alloy is anodically polarized with a current density which ranges from about 10 to about 500 A/m².
 21. The method according to claim 1 wherein the solution has a temperature which ranges from about 10° C. to about 40° C.
 22. The method according claim 1 wherein the cleaning and passivating steps are performed in a time period ranging from about 10 seconds to about 5 minutes.
 23. The method according claim 1 wherein the light-metal alloy surface has not undergone a preliminary chemical treatment prior to the cleaning and passivating steps.
 24. The method according claim 1 wherein the light-metal alloy is a regenerated material.
 25. A method for coating a metallic surface comprising the steps of: cleaning a surface of a light-metal alloy with a solution comprising phosphoric acid and an alcohol, wherein the alloy is anodically polarized; passivating the surface in an oxidizing step; and coating the surface after the cleaning and passivating steps.
 26. The method according to claim 25 in which the coating is carried out by means of electroless metal plating. 